Is Tantei Opera Milky Holmes: Summer Special hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

TV Special · 2011 · 1 episodes · PG-13 - Teens 13 or older
Comedy, Mystery, Super Power
A special program of Tantei Opera Milky Holmes produced by the staff of the TV series. After some pushing by Irene and her father and a lack of support otherwise, Kokoro stubbornly quits her job as a police detective to become an idol. She soon finds the idol life draining and starts to miss her old detective life. When Henriette notices Sherlock's loneliness without her, she stages a statue theft in order to convince Kokoro to return to the force. (Source: AniDB)
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